Method of detecting and eliminating flaws in solid material of high molecular order



Jan. 3, 1961 H. o. KOCH 2,967,240

METHOD OF DETECTING AND ELIMINATING FLAWS IN SOLID MATERIAL OF HIGH MOLECULAR ORDER Filed March 7, 1958 FIG R w W W Hans Ofio Koch A TT'ORNE rs United States Patent Ofifice 2,967,240 Patented Jan. 3, 1961 METHOD OF DETECTING AND ELIMINATING FLAWS IN SOLID MATERIAL OF HIGH M- LECULAR QRDER Hans Otto Koch, Mozartstr. 29, Bonn, Germany Filed Mar. 7, 1958, Ser. No. 719,718

Claims priority, application Germany Mar. 9, 1957 Claims. (Cl. 250-53) This invention relates to the detection and elimination of faults in crystalline and analogously constructed materials of high molecular order, especially piezoelectric monocrystals and dielectric crystals having structurally conditioned electrical conductivities, such as crystals of quartz and zinc blende.

One of the objects of this invention is to provide a novel method for the detection of faults in the atomic arrangement of piezoelectric and dielectric crystals of qualified electrical conductivity, such as prepared quartz or zinc blende crystals, to permit inter alia of the segregation of useless crystals from useful ones.

Another object of this invention is to provide a novel method for the elimination, so far as may be possible, of specific faults in such crystals with a View to improving substantially the mechanical, chemical, and electrical properties thereof.

It has been found in the course of earlier investigations that the discrepancies between the theoretical and practical mechanical strengths of monocrystals are conditioned by the presence of structural defects which may be regarded as extremely small internal cavities at internal boundary faces in the crystal, and commonly and in general referred to as flaws.

A knowledge of the effect, of such flaws upon the properties of a crystal is of technological significance and a matter of paramount importance in the further development of the art. So far, there has been no reliable method of detecting these faults and of controlling the production of a technologically desirable high degree of internal order in the arrangement of the smallest structural elements and of maintaining them in this order.

Although various methods are already known for achieving a limited improvement in the surface and internal structure of crystals, none of them, in the absence of the means for detecting and eliminating the primary causes that underlie faults that occur in the material, is able to ensure a permanent improvement of molecular order.

It is therefore more specifically an object of the present invention to provide a novel method of detecting and eliminating faults in crystalline and analogously constructed materials which are in a state of high molecular order, and especially in piezoelectric monocrystals and dielectric crystals with structurally conditioned electrical conductivities (such as quartz and zinc blende crystals) and this novel method consists in simultaneously exposing the surfaces of crystal plates to the influence of an electrical field (preferably one of high intensity) and also if desired to the effect of a caustic electrolyte and thereafter subjecting said crystal plates to the influence of radiation, such as X-rays or electron radiation, capable of producing interference phenomena. X-rays and electron radiation are hereinafter embraced in the general term radiation. The ejection of foreign ions by the proposed electrolytic treatment can be verified spectroscopically.

A further feature of the invention is the verification of microscopically observable phenomena by electrochemical means, whereas the detection of flaws in the atomic arrangement and at the same time the verification of the healing effect produced by the electro-chemical method is effected by the observation of interference phenomena.

The elimination of flaws in the atomic crystal structure for the purpose of improving important properties, such as mechanical strength, chemical resistance, and electrical insulation as well as the piezoelectric constants and the stability of the frequency of resonant quartz crystals is practically already achieved by the performance of the electro-chemical method the invention proposes to use.

However, coarser faults in the orientation of small monocrystalline re ions must be eliminated by subsequent irradiation by X-rays.

The results that can be secured by the method proposed by the invention provide a substantial improvement of numerous properties of the material and quite generally widen the field in which crystalline substances can be employed.

In order that the invention may be the more readily understood the method of detecting faults, that is the method of testing, and the method of eliminating the faults, that is the method of refining the crystal, will be separately hereinafter described.

Figure 1 shows diagrammatically the diffraction of electrons in crystal plates of the size of 1 cm. in the reflection process.

Figure 2 illustrates diagrammatically the roentgen- Laue-process with a refined pickup camera for accomplishing the irradiation process, and

Figure 3 is a diagrammatic view illustrating the etching process in the electrical field for the detecting and prevention.

In Figure 1, the photo plate is shown at 1 and 2 is a swinging and adjusting device. Crystal sample 3 is positioned on the adjusting device and a source of electrons is indicated at 4. A vacuum pump is shown at 5.

InFigure 2, 6 is a Roentgen tube having an anode 7 and a heated cathode S. A gimbal swinging adjusting device is shown at '9 and a long narrow tube screen at 10. The photographic film 11 is positioned in a light, tight, protective covering of paper and the crystal plate 12 is positioned on the side which is turned towards the film in front of the opening of the screen.

In Figure 3 a high-tension device 13 is shown which may be used with alternating current or for the intermittent direct current or for the real direct current. The terminals 14 are for the power supply voltage and 15 is the double etching cuvette. The electrical wires 16 lead to the cuvette. The crystal plate is shown at 17 and the graphite electrodes at 18.

In the performance of the novel method of testing according to the invention, a crystal specimen is etched and simultaneously exposed to a powerful electrical field. An alternating field is used for the purpose of producing symmetry conditioned fissuration phenomena which are characteristic of the particularly frequent and pronounced flaws on and immediately below the surface of the tested crystal. On the other hand, the application of a unidirectional field reveals perceptible differences in the appearance of the etched surface, due to the directional migration of undesirable foreign impurities out of the crystal under the influence of the field and the dis solving action of the chemical.

In other words, the electro-chemical treatment produces various results. The flaws of internal boundary surfaces are increased in size by the mechanical effect of a powerful alternating electricalfield orof a pulsating unidirectional field or alternatively of a non homoge'n'eous static unidirectional field assisted by the etching effect. Locally strong field forces tend predominantly to affect non-homogeneities in the crystal structure. Since such locations are mainly found on the surfaces of principal growth, where for instance impurities have been precipitated, a microscopically observable crystallographically orientated fissuration can often be found at such points, which resembles natural cleavages, and which (for instance, in zinc blende) is found to be identical therewith.

On the other hand, natural quartz exhibits no significant cleavability so that the fresh fissuration described can be regarded as being the consequence of an artificially improved cleavability, brought about by the effects of a method practised according to this invention. As a matter of fact, in both instancesin the cleavage of most natural crystals, as well as in fissuration which is electrically producedthe underlying reasons are the same, namely the existence of internal boundaries representing specific structural faults which are a characteristic feature of the so called microstructure of the natural crystal.

Consequently the fissuration produced by the method according to the invention may serve as an indicator for the presence of internal crystalline faults which under high mechanical loads may give rise to pressure twinning.

The difference in penetration and nature of the chemical solution process when etching in a unidirectional field brings out the growth relief and the fault structure. This permits faults in orientation of individual microscopically small crystal elements (so-called internal intergrowths) to be recognised. Detection is inter alia by observation of the different ways in which solution of the crystal substance proceeds owing to the orientational characteristics of the crystal, further modified in the electrical field by the conduction of ions. The electrolytic ejection of disturbing ions is most pronounced, for instance in a quartz crystal, if the direction of the field is arranged to be parallel with the c-plane of the crystal, so that the de velopment and configuration of etched patterns induced by the crystal faults will allow conclusions to be drawn as to the frequency of faults in the crystal.

The elimination of the foreign impurities responsible for the flaws in the monocrystal, such as of alkali ions in quartz, by causing these ions to migrate in the unidirectional field to the cathode, can be verified, by a method according to this invention, at the end of the electrolytic treatment-for instance, by spectroscopic analysis of the remaining etching liquid in respect of foreign impurities that have accumulated at the electrodes.

Naturally, the elimination of the impurities can also be verified by checking the improvement obtained in the properties of the crystal, such as an increase in mechanical strength.

The invention further proposes to supplement the electro-chemical method of testing the crystal by testing methods which consist in exposing the crystal to radiation that is capable of producing interference phenomena.

An interferential method which makes use of X-rays may preferably be used for examining the interior of the crystal, whereas electron interference phenomena will allow conclusions as to the condition of the crystal surface.

The X-ray interference test may be performed according to the invention by means of a Laue type camera provided with a tubular stop which permits of very little scattering of the beam and hence gives high resolution. This apparatus reveals natural faults in the crystal structure as well as fault phenomena engendered by the electro-chemical treatment.

On the one hand, these may consist in symmetry changes which the Laue method is especially suitable to detect. For instance, in the case of a quartz monocrystal even very minute regions of natural or artificial pressureproduced twinning can still be detected, a result which is usually impossible to achieve with conventional Laue type cameras which have a wide tubular stop.

On the other hand, the shape and the more or less incomplete development of the X-ray reflexes permit of reaching conclusions as to the nature and the elimination of the crystal faults, but quantitatively useful results can be secured only if a camera of high resolution is available. The disadvantage accruing from the employment of a very narrow and long tubular stop on the Lane camera, namely that of increasing the necessary exposure time of the film to several hours, is more than offset by the advantage gained of higher resolution and better definition of the reflexes.

In spite of the known difliculties due to electrical charging phenomena which arise when employing the method of electron diffraction, this can be successfully performed if, in accordance with the invention, the back reflection technique is applied to the surface examination of, for instance, quartz crystals that have been refined.

The following further data are intended to illustrate the method of testing proposed by the present invention:

For instance, for performing the electro-chemical method, a polished sample in the shape of a plate 1 mm. thick is cut out of the crystal that is to be tested in a certain orientation and then cemented in to a V-shaped twin etching vessel made of acid-resistant material, such as polystyrene or the like, where it forms a condenser dielectric. Two preferably acid-resistant electrodes, for instance of graphite, are immersed on each side in the caustic contained in the twin vessel. For etching quartz, a 20 to 40% by vol. hydrofluoric acid is suitable and for zinc blende a concentrated hydrochloric acid may be used. An alternating potential of 50 cycles and of at least 7 kv. peak voltage is applied to the electrodes. This produces microscopically visible fissuration on the surface of the quartz base. For examining the prismatic and rhombohedral surfaces of a quartz crystal a peak voltage of at least 20 kv. and an acid concentration not exceeding 20% is required to produce fissuration.

For examining the base and rhombohedral sections of quartz for growth zones and layers the procedure is substantially the same as described. However, instead of applying an alternating potential a pure continuous potential of at least 8 kv. must be applied. The necessary acid concentration depends upon the desired etching effect and generally a 10 to 40% hydrofluoric acid will be appropriate.

When tubular stops of 0.3 mm. internal diameter are used on the Laue camera in using the method of X-ray examination according to this invention, the irradiation of individual fissuration figures on the quartz base produces noticeable intensity shifts in the Laue reflexes, in addition to the optically double-refracting strain halos around the fissure stars, so that the trigonal symmetry of untwinned low temperature quartz comes perceptibly close to a hexagonal symmetry. Since hexagonal quartz is unstable except at temperatures above 573 C., this indicates transition in minute regions into the Dauphine twin structure. This may be explained as being due to pressure twinning as a result of electrically conditional mechanical strains which arise where flaws are contained in the crystal. Quartz base plates, for instance 0.4 mm. thick, are used for photographs of this kind. For reference purposes a polished untreated crystal, for instance, a quartz base specimen, may be photographed, then electro-chemically treated, and the same point on the specimen photographed once again. Artificial thermal twinning, i.e. Dauphine type twinning can, for reference purposes, be induced by heating the crystal to over 600 C. and then slowly cooling it. By comparing these photographs the structural fault phenomena can be examined, and by analysing them in the conventional manner the structure of the crystal in its original state and its crystallographic state after electro-chemical treatment can be ascertained. Since the irradiation treatment also eliminates the faults, the same X-ray technique (employing fine long stops on a Laue camera) can be used, owing to its better resolution, for judging and checking-in the case of the different faults-the changes in the crystal brought about by the electrolytic refining process and the radiation healing technique. All these photographs are preferably taken with a camera equipped with a tubular stop of 0.3 mm. internal diameter. The crystals photographed after they have been subjected to the electrolytic constant potential treatment show a per ceptible decrease in the diameter of the reflexes as a result of the electrolytic treatment. This idealisation of the Lane reflexes can be very nearly quantitatively evaluated and improves as the continuous potential treatment is continued and further X-ray photographs are taken. Tubular stops of 0.5 mm. internal diameter will also produce good results.

The photographs obtained further reveal that disturbances in orientation in small crystal regions are already eliminated after a brief irradiation by X-rays. Back reflection photographs will also permit the location of the faults to be determined. Owing to the shallow depth of penetration of the X-rays in the back. reflection technique, disarranged crystal regions which disappear during X-irradiation belong to disturbances of the surface layer due to the grinding and polishing of the crystal specimens.

The detection of faults by electron diffraction according to the invention is performed, for instance, on quartz plates 1 cm. thick by the back reflection technique with an anode constant potential of between 50 and 60 kv. in a vacuum of only about torrs. The photographs may be taken at a very slanting angle of incidence on a polished quartz base plate and on etched quartz base surfaces. The specimens are rotated about the Y-axis in the same way as in the rotating crystal method.

Apart from discrete point reflexes the photographs then show either distinctive Kikuchi lines, which indicate a substantially uniform, nearly ideal crystal lattice on the surface, or circularly arcuate Debye-Scherrcr rings consisting of distorted point reflexes, which are indicative of a texture-like orientated roughening in a certain preferential direction. Care must be taken to adjust the beam accurately in the x-direction. Generally speaking, an optimum in photographic reliability can be achieved by variation of the angle of incidence.

The diffraction method therefore permits of the simultaneous detection of residual parts of the dissolved surface layer and the examination of the improved substrate freshly obtained by the electrolytic treatment. Conclusions thus drawn may be substantiated by the results of conventional electron microscopy with the help of lacquer impressions.

For the spectroscopic detection of the impurities that have been ejected by the electrolytic treatment, the impurities which have accumulated in the hydrofluoric acid at the negative electrode after the quartz plate has been treated are rendered amenable by absorbing the acid in analytically pure carbon powder. Before starting with the electrolytic crystal purification quantitatively clean working conditions must therefore be established to permit the analytical samples to be later obtained. The baths, electrodes, and crystal faces must therefore be mechanically and chemically cleaned as throughly as possible, and the hydrofluoric acid used must be of analytical purity. Samples should be previously taken of the acid and of the distilled water, and these (as well as all accessories) spectroscopically examined to ensure that alkalis subsequently presumed to have been extracted from the crystal cannot have entered from the outside.

The above described methods permit the state of the crystal structure to be adequately determined so as to give substantial assistance in judging and creating controllable crystal properties in accordance with technological requirements.

In the refining treatment which eliminates the faults the crystal surface is simultaneously exposed to the in fluence of a unidirectional electrical field of considerable strength and possibly to the influence of a caustic electrolyte. In the elimination of atomic faults in quartz crystals, etching is performed in a unidirectional field in such manner that a field component of suflicient magnitude will be set up for inducing a current in the direction of the c-axis. This means that the faces of the crystal plates used must. not be cut parallel with the c-axis since the method Would then become ineffective. The process of refining relies in the electrolytic purification of the crystal lattice by the removal of foreign impurities contained in the crystal. A stable condition of the lattice, that is, the natural lattice that had been previously disturbed by the presence of the foreign ions, will arise when by electrolytically assisted self diffusion of oxygen, for instance in a quartz lattice, the unoccupied Frenkel oxygen positions are filled. In this way the lattice may approximate an ideal lattice. The resultant improvement in crystal properties, such as the increase in mechanical strength, as well as in chemical and electrical resistance, can be experimentally verified, for instance by the negligible effect of electrical and caustic attack on the electrolytically treated crystal faces.

According to the invention the crystal specimens can be further improved by a radiation refining process" which consists in exposing them to X-irradiation which irrespectively of crystal orientation supplies energy for the reorientation of very unstable intergrowth regions in the crystal lattice into the general monocrystalline scheme.

The following particulars serve to illustrate this further refining process:

To drive as many disturbing ions out of the crystal lattice in as short a time as possible by the constant voltage-etching method, as high as possible a field strength is generated, up to a maximum of 20 kv./mm. With an acid concentration of 20 or 40% for rhombohedral or base sections respectively, the high voltage is allowed to act for at least two hours. To avoid strong etching of the crystal surface opposite the electrode, H O may, if necessary, be used at the cathode instead of hydrofluoric acid.

The refining irradiation treatment is performed by exposure to intense X-rays, supplied for instance by an X- ray tube with a tungsten anode operating at 50 kv. or more. For the rapid introduction of the necessary radiation energy the crystal specimen may be placed as closely as possible to the Lindemann window and exposed to the full beam of the tube. The exposure time required for the reorientation of unstable disarranged crystal regions may then be reduced to a few minutes. According to the nature of intergrowth the treatment may successfully be performed either before or after electrolytic refining. Photographs show that without further action disorientation has entirely disappeared at the end of 50 hours irradiation by an X-ray tube with a tungsten anode operating with a sinusoidal 5O cycle voltage of 50 kv. (effective).

The total treatment according to the invention substantially improves important physical properties of the crystals and thus constitutes a substantial advance in the art of producing piezo-electric quartz, osciilator and resonant plates.

I claim:

1. Method of detecting and eliminating flaws in solid material of high molecular order, which comprises the simultaneous steps of exposing the surface of a plate of such solid material to the influence of an electric field and to the influence of radiation capable of producing interference phenomena, said solid material plate being also subjected to the effect of a caustic electrolyte and the elimination of flaws being vertified by subsequent examination by spectroscopic analysis.

2. Method as claimed in claim 1, characterised in 7 that the high intensity field is an alternating field.

3. Method as claimed in claim 1, characterised in that the high intensity field is a unidirectional field.

4. Method as claimed in claim 3, more particularly for eliminating faults in a quartz crystal wherein a large component of a unidirectional electrical field is applied with consequent generation of an electrolytic current along the c-axis of the quartz crystal.

5. Method as claimed in claim 1, wherein said solid material is exposed to irradiation by X-rays and the X- ray interference pattern recorded photographically.

6. Method as claimed in claim 5, using a Laue X-ray camera fitted with a narrow tubular stop to minimise divergence of the beam.

7. Method of detecting and eliminating flaws in a solid material of high molecular order, which comprises the steps of exposing the surface of a plate of such solid material simultaneously to the influence of an electric field and to the effect of an electrolyte, and also exposing said surface to the influence of radiation capable of producing interference phenomena.

8. In a method of detecting and eliminating flaws in a solid material of high molecular order, the step of simultaneously exposing the surface of a plate of such solid material to the influence of an electric field and to the effect of an electrolyte.

9. Method as claimed in claim 1, wherein the plate is exposed to an electron beam, the resultant electron beam diffraction pattern in the reflected beam being recorded photographically.

10. Method of detecting and eliminating flaws in a quartz crystal plate which comprises the steps of exposing the surface of the crystal plate simultaneously to the influence of a unidirectional electric field of high intensity and to the effect of a caustic electrolyte, and subjecting said plate also to irradiation by X-rays, and photographing the X-ray interference pattern by means of a Laue camera adapted for minimising divergence of the beam.

11. In a method of detecting and eliminating flaws in a crystal having an axis of symmetry preferably fit for electrical conductivity like c-axis of quartz the step of exposing a surface of a plate of such crystal to the influence of a unidirectional electrical field of high intensity and simultaneously to the effect of a caustic electrolyte, whereby an electrolytic current is generated along said c-axis.

12. Method of detecting and eliminating flaws in a crystalline or another solid having a structure of high molecular order comprising the steps of exposing the surface of a plate of said crystalline or similarly built solid simultaneously to the influence of an electric field and to the effect of a caustic electrolyte, and also to the influence of radiation capable of producing interference phenomena.

13. Method of detecting and eliminating flaws in a crystal or similarly built plate having a structure of high molecular order which comprises the steps of exposing the surface of the crystal plate simultaneously to the influence of an alternating electric field of high intensity and to the effect of a caustic electrolyte, and also to the influence of X-rays.

14. Method of detecting and eliminating flaws in a quartz plate which comprises the steps of exposing the etched surface of the quartz crystal plate to the influence of an unidirectional electric field of high intensity, whereby an electrolyte current is generated along the c-axis of said plate, and some time ago or simultaneously or afterwards subjecting said plate to the influence of radiation that is capable of producing interference phenomena and of pushing off mechanical tensions in the crystal.

15. In a method as claimed in claim 14, exposing the quartz crystal plate to irradiation by X-rays and recording the X-ray interference pattern photographically.

References Cited in the file of this patent UNITED STATES PATENTS 2,380,236 Harker July 10, 1945 2,437,912 Frondel Mar. 16, 1948 2,437,913 Frondel Mar. 16, 1948 2,437,914 Frondel Mar. 16, 1948 2,457,555 Haworth Dec. 28, 1948 2,468,301 Mason Apr. 26, 1949 

